The present invention relates to a transducer that employs a field emitter array as a source of electrons and is suitable for use in the submillimetre wavelength region of the electromagnetic spectrum and to a method of making the same. Particularly, but not exclusively, the present invention relates to a generator that is capable of producing signals in the 300 GHz to 10 THz frequency region of the electromagnetic spectrum. This region comprises the submillimetre wavelength region (300 GHz-3 THz) and part of the far infrared region (3 THz-10 THz), but is loosely termed the “terahertz” region.
The 300 GHz to 10 THz spectral range lies at the cross-over between conventional electronics and optics and to date fundamental sources of power in this spectral range have been expensive and bulky. This has limited technology operating in this spectral range to applications in specialist areas for examples solid state science and astronomy. No low-cost, compact, convenient fundamental sources exist and this paucity of sources has given rise to the term “terahertz gape”.
Conventional generators capable of generating signals at the lower end of the terahertz gap are generally transit-time devices such as bipolar transistors or Gunn oscillators. However, such devices are small and difficult to fabricate and are only capable of generating low power due to fundamental considerations of quantum mechanics and currently insurmountable technical fabrication difficulties. At the upper end of the terahertz gap optical sources are usually employed. However, these too are inherently weak sources and are subject to fundamental physical limitations. All-optical generation of terahertz pulses has recently been demonstrated, see for example “T-Ray Imaging” by Daniel Mittleman, Rune H Jacobsen and Martin C Nuss in Journal of Selected Topics in Quantum Electronics 2(3) p 679 and generation of terahertz pulses by multiplication of fundamental frequency sources, X Melique, C Mann, P Mounaix et al IEEE Microwave and Guided Wave Letters, 8 384-386 (1998) has also been described. However, the expense and/or size of the equipment described in these articles means that they are unsuitable for wider use in commercial or mass-market applications. Thus it can be seen that research into generators for this range of frequencies has focussed on developing unique devices utilising physical characteristics specific to these frequencies and research continues into pushing laser frequencies further into the far infrared.
Field emitter arrays are increasingly being used in a wide variety of applications. The use of a field emitter array as part of a generator is known but only in relation to generators operating at the much lower microwave frequencies. For example an article entitled “Application of Gated Field Emitter Arrays in Microwave Amplifier Tubes” by S G Bandy, M C Green, C A Spindt et al in Proc. 11th Int. Conf. On Vacuum Microelectronics, p132 describes the use of a field emitter array to provide electrons for a conventional klystron-type structures milled from metal using conventional techniques. However, this generator is only capable of generating signals around 10 GHz, well outside the terahertz gap in the far infrared.
There are, however, many advantages to systems operating in the terahertz gap. For imaging systems, for example, the advantages include the comparability of the possible resolution obtainable with the size of everyday objects and the ability of the radiation to penetrate many commonly-used materials or substances such as plastics, foodstuffs, fabrics, teeth and human skin. Terahertz radiation (unlike x-ray radiation) is non-ionising and therefore presents none of the hazards directly arising from ionising radiation. Advantages for other types of application, for example communication systems, are described in “New Directions in Terahertz Technology” by J M Chamberlain, Kluwer, 1997.
There is therefore a need, that is currently not being met, for a new terahertz generator capable of generating signals in the terahertz gap that is suitable for commercial applications and for mass production.